ArduIMU Quadcopter

Is this the quadrotor week? :-)
Hi all, on Christmas holidays I have started my DIY project to construct a cheap (but good performance) quadcopter based on ArduIMU hardware only.

I have used standard props (NO counter rotating) that are easy to get and if we want construct the quad on other size you have no problems finding the right prop. To compensate for torque I mount front and rear motor with a small angle (about 6º, see Photos). I first use this concept on this project (http://www.rcgroups.com/forums/showthread.php?t=297067) six years ago!!...

Note how small seems the ArduIMU PCB on this quad! and how clean is this setup. We can construct really tiny quads this way and if we use standard props we have a wide range of motors and props to choose...

Development:
The first test I did on this project was the vibration test of the ArduIMU with motors running and with the new firmware (I use r20) there are no problems with this. Because we have 4 motors running there are more noise on accelerometers (it´s normal) so I added a new low pass filter on the accels and also adjust the gains for drift corrections...

The main loop with IMU code and PID controls for roll, pitch and yaw runs at 70Hz and the PWM outputs for ESC´s have a 125Hz rate. As always, adjust the PID gains is a very important task . D term is really important for a quad and I am using directly the raw gyro readings (bias corrected) to feed the D term in order to make it more responsive.

The most difficult part was the developing of the function to generate the PWM pulses for the ESC´s because we need 4 PWM servo outputs with very good precision (1-2us) and at a high rate (now is 125Hz).
This code generate more than 10,000 interrupts per second (only ADC generates more than 8,800) so in this enviroment is difficult to generate precise pulses.
The solution was to use the Timer2 overflow interrupt and a "trick" based on Timer 1 counter readings to achieve this high precision. I have also one (or two) more standard servo output for future applications (camera stabilization?)
The radio input is done via the ICP pin and interrupts so high precision is achieved (1us). We need to feed a PPM signal (directly from Rx or via the new Jordi´s PPM encoder...)

On the future we can use the magnetometer of ArduIMU, but now the yaw is like having a Heading hold gyro (note that quads have slower yaw response than helis)

This is the first stage on the development process of this project but I have made some tests with good results.

Thats really nice !
You have to have a magnetic sensor and pressure sensor on board for better flight control.
Here is one such open source Quadrocopter design known as UAVX by Prof. GK Egan;http://code.google.com/p/uavp-mods/updates/list

You have used same propellers and not counter rotating one by adjusting the angle of two motors slightly tilted, this idea is just superb !Hats off to you for such a simple but great idea implementation as it is really difficult to get counter rotating props.

The problem with it was the angles had to be superbly accurate or it would get unbalanced. Any hard landing would throw them off. Unless you had a fully servo gimbaled propeller, it was easier to use square tubing & maxxprod propellers.